This invention relates to double-beam spectrophotometers, and more particularly, to spectrophotometers capable of spectral analysis in the infrared region using a thermal infrared detector such as a vacuum thermocouple detector.
As is well known in the art, the double beam spectrophotometers measures the transmittance of a sample by allowing light to alternately enter the sample and a reference or standard material (or an empty cell), measuring the intensity of a sample beam that has passed the sample and a reference or standard beam that has passed the reference or standard material, and comparing the sample beam intensity with the reference beam intensity, with the resultant ratio giving the transmittance of the sample. The signal processing method in such double beam spectrophotometers, that is, the method of automatically outputting the transmittance of the sample is generally classified into the optical null balance system widely used in conventional infrared spectrophotometers, the electrical direct ratio system, and the automatic gain control system, of which the latter two were recently developed.
The optical null system measures the transmission of a sample by alternately switching a sample beam and a reference beam attenuated by a mechanical beam attenuator, taking out an AC signal having an amplitude proportional to the difference (I.sub.0 -I) between the reference beam intensity I.sub.0 and the sample beam intensity I, using this signal as an error signal in a closed loop, thereby automatically adjusting the mechanical attenuator associated with the reference beam such that the value of the difference (I.sub.0 -I) may always become zero. When the attenuator is adjusted in this way, the magnitude of beam attenuation itself, that is, the distance of movement in the attenuator is proportional to the transmittance so that changes in the transmittance of a sample can be recorded by recording the movement of the attenuator.
The above-mentioned optical null system is best with respect to percent utilization of signals and stability of measurement, but has several drawbacks described below. First of all, since the accuracy of transmittance largely depends on the mechanical accuracy of the attenuator itself as well as the associated drive system, the spectrophotometer is difficult to exhibit highly accurate and stable performance. In connection with this, due to fluctuations in rotation of a driving servo motor for moving a wedge-shaped stop commonly used in the attenuator, error in linearity of a potentiometer for detecting the position of the wedge-shaped stop, and other factors, the distance of movement of the stop is not always proportional to the magnitude of attenuation, often resulting in low accuracy of transmittance measurement. Further, the inclusion of the optical system in the servo loop results in a complicated and expensive apparatus which handles signals in a complicated way and has poor response. In the case of a sample having high absorbance, the sample beam intensity approximates to zero, and accordingly, the reference beam intensity is also attenuated to a level near zero, resulting in reduced loop gain and reduced reliability. In addition, the attenuator itself is reduced in accuracy when the magnitude of attenuation is very high, that is, when the sample has very high absorbance. These undesirably causes a substantial reduction in accuracy of measurement of a high absorbance sample. When the sample beam intensity is zero, the reference beam intensity also becomes zero so that the loop gain is zero, rendering the apparatus inoperative. It is, also a serious drawback that calibration, adjustment, and inspection of the apparatus cannot be carried out under the standard condition having the sample beam intensity set to zero.
The automatic gain control system which does not use a beam attenuator is a system in which time division is made by beam path switching means such as a sector mirror to allocate fractions to sample and reference beams and dark state when both sample and reference beams are interrupted, and the gain of signal processing means, for example, the gain of a detector or an amplifier is automatically controlled such that the reference beam intensity may always be at a constant level. In this system, an output of the amplifier corresponding to a sample beam directly corresponds to the ratio of sample beam intensity to reference beam intensity, that is, the transmittance of the sample. The transmittance is then directly available simply by sample holding an output of the amplifier corresponding to a sample beam. This system uses a gain controllable detector, for example, photomultiplier as the detector. When the detector gain is controlled in a feedback manner, all electric signal systems are contained in this loop so that the linearity and stability of detector and amplifier have no influence on measurement, insuring high accuracy in measurement. Further, because of the absence of a beam attenuator and a mechanical servo system and the possibility of setting absolute zero, this system has eliminated most of the drawbacks of the optical null sytem. However, infrared spectrophotometers carring out spectral analysis in the infrared region must use thermal detectors such as thermocouples. The thermal detectors are difficult to control their sensitivity and thus incompatible with the automatic gain control system. A spectrophotometer may be constructed using a thermal detector such that the thermal detector is connected to control the gain of an amplifier. However, since the thermal detector has a considerably slower speed of response and a considerably larger time constant than a photomultiplier detector used in visible-to-ultraviolet spectrophotometers, and the output waveform of the thermal detector does not correspond to the waveform representative of changes of the beam which has passed the beam path switching means, it is difficult to devise a practical automatic gain control system using a thermal detector.
The electrical direct ratio system does not use a beam attenuator and is a system in which an output signal of a light detector containing in admixture components corresponding to the intensities of sample and reference beams is amplified by a common amplifier before the individual components are electrically separated and the ratio of the individual components is electrically determined. This system is generally subdivided into two systems, frequency component detection system and phase discrimination system, depending on how to separate and take out components representative of sample and reference beam intensities from the amplifier output. In either case, separation of signal components can be carried out using thermal detectors such as thermocouples. Consequently, this system is adaptable to infrared spectrophotometers.
Among prior art spectrophotometers of the electrical direct ratio system, specifically the frequency component detection system is one disclosed in Japanese Patent Application Kokai No. SHO 52-10790 (published on Jan. 27, 1977). This spectrophotometer uses as beam path switching means for interrupting and switching beam paths for sample and reference beams, a sector mirror capable of alternately discontinuing the sample and reference beams at a given frequency f and a chopper capable of discontinuing the sample and reference beams at a frequency 2f twice the frequency f of the sector mirror. Since a component having frequency f in an output signal of the detector corresponds to the difference (I.sub.0 -I) between the reference beam intensity I.sub.0 and the sample beam intensity I, and a component having frequency 2f corresponds to the sum (I.sub.0 +I) of the reference beam intensity I.sub.0 and the sample beam intensity I, an output proportional to I.sub.0 is obtained by adding the components having frequencies f and 2f and another output proportional to I is obtained by subtracting the one component from the other component. The value of I/I.sub.0 is readily obtained through arithmetic operation on the ratio of these outputs. In this process, even when a sample beam is shut off or becomes zero, neither of the signals representative of the components having frequencies f and 2f become zero in the signal processing route. The apparatus is thus difficult to carry out calibration, adjustment or inspection under the standard condition having the sample beam intensity reduced to absolute zero. A serious problem arises that calibration and inspection of the apparatus cannot be carried out readily and accurately.
Among prior art spectrophotometers of the electrical direct ratio system, specifically the phase discrimination system, is one disclosed in Japanese Patent Application Kokai No. SHO 57-52832 (published on Mar. 29, 1982). This spectrophotometer is adapted to take out sample and reference beams at a phase difference of 90 degrees by means of a sector mirror. The detector thus produces an output signal in which a component representative of the sample beam intensity and a component representative of the reference beam intensity are mixed with a phase difference of 90 degrees, the output is synchronously rectified to provide a DC signal corresponding to the sample beam intensity and another DC signal corresponding to the reference beam intensity, and then the ratio of these signals is determined by arithmetic operation. Nevertheless, as wavelength scanning is generally carried out in spectral analysis, an output waveform of the detector in one cycle sometimes loses its symmetry in a wavelength region where the sample shows high absorbance. In addition, absorption by atmospheric water vapor and carbon dioxide has probably an influence on both the reference and sample beams, distorting the output waveform of the detector in one cycle. In the case of the phase discrimination system, such a distortion of the output waveform largely affects the phase, eventually resulting in a significant error in measurement. This problem will be further explained below. For high absorbance samples, absorbance varies very rapidly during wavelength scanning, resulting in a graded sample beam intensity in one cycle. Since a detector, particularly a thermal infrared detector such as a thermocouple has a large time constant, its output signal appears to have the effect of integrating the intensity waveform of incident light. If the sample beam intensity in one cycle has a gradient as mentioned above, the output waveform is offcentered from the input waveform of the sample beam intensity so that the phase difference between the sample and reference beam intensity components in the output is shifted from 90 degrees, resulting in an error. Also, absorption by atmospheric water vapor and carbon dioxide has an influence on the intensity waveform of incident light to the detector to distort an output signal of the detector to give rise to a similar phase shift, resulting in an error in measurement.
In the case of the phase discrimination system spectrophotometer, if dust deposits on slits in the paths for reference and sample beams and any obstructions in proximity to the beam paths partially intercept beams or mechanical positioning of slits is inaccurate, then the rise or fall of a waveform representative of the intensity of an incident beam to the detector is shifted from the originally set phase, and thus the corresponding output waveform of the detector is deviated, also resulting in an error in measurement. In principle, the phase discrimination system has the advantage that when a sample beam is zero, the DC signal which is separated by synchronous rectification as corresponding to the sample beam intensity essentially becomes zero so that calibration and inspection can be carried out under the standard condition of absolute zero. However, if the phase is shifted for any one of the above described reasons, the DC signal corresponding to the sample beam intensity is somewhat deviated from zero, reducing the accuracy in calibration and inspection.
As described above, the signal processing systems of the prior art spectrophotometers have their own advantages and disadvantages and are unsatisfactory particularly for spectral analysis in the infrared region.
It is, therefore, an object of the present invention to provide a spectrophotometer which has eliminated the disadvantages of the various prior art systems, and is particularly suitable for spectral analysis in the infrared region at a high accuracy with minimal error while calibration and inspection can be carried out in the standard condition of absolute zero.
Making extensive investigations to improve the electrical direct ratio system, specifically the frequency component detection system such that the standard condition of absolute zero can be used for calibration and inspection, the inventors have found that the standard condition of absolute zero can be used for calibration and inspection by using a specific order and combination of sample beam, reference beam, and dark state rather than those used in the prior art system as described in Japanese Patent Application Kokai No. SHO 52-10790, and by modifying the frequency component detection system to meet therewith.